In microwave-assisted chemistry, microwaves are used to initiate, drive, or otherwise enhance chemical or physical reactions. Generally, the term “microwaves” refers to electromagnetic radiation having a frequency within a range of about 108 Hz to 1012 Hz. These frequencies correspond to wavelengths between about 300 cm to 0.3 mm Microwave-assisted chemistry is currently employed in a variety of chemical processes. Typical applications in the field of analytical chemistry include ashing, digestion and extraction methods. In the field of chemical synthesis, microwave radiation is typically employed for heating reaction materials, many chemical reactions proceeding advantageously at higher temperatures. In addition, when pressureriseable reaction vessels are used, many analytical or synthetical processes can be further enhanced by increasing the pressure in the vessel. Further, when, for example, digestion methods for analytical purposes are used, the generation or expansion of gases inside the vessel will necessarily increase the internal pressure. Thus, in order to ensure that no reaction products are lost for subsequent analysis, vessels must be used which are able to withstand high internal pressures in these cases.
Usually, most microwave-assisted reactions are performed in open or, preferably, in sealed vessels at temperatures rising up to 300° C. Typical pressures range from below atmospheric pressure, e.g. in solvent extraction processes, up to 100 bar, e.g. in digestion or synthesis processes.
Microwave-assisted chemistry is essentially based on the dielectric heating of substances capable of absorbing microwave radiation, which is subsequently converted into heat.
Many apparatuses and methods currently employed in microwave-assisted chemistry are based upon conventional domestic microwave ovens operating at a frequency of 2.45 GHz. As magnetrons operating at this frequency are produced in large quantities for domestic appliances, microwave apparatuses for microwave-assisted chemistry using such magnetrons can be manufactured at relatively low cost.
In many applications, such as analytical chemistry and chemical synthesis, uniform heating of the samples is of utmost importance since, for example, reaction rates strongly depend on the temperature of the sample.
When heating samples by microwave radiation, pressurized sample vessels are often employed to increase the speed of the reaction and/or to increase the yield of the reaction. In order to fully benefit from the use of pressurized vessels or containers, it is important to ensure uniform reaction conditions throughout the sample. In prior art, it has therefore been suggested to control pressure and/or temperature in the sample vessel. It is also known to employ motorized stirrers or magnetically driven stirring elements to ensure uniform heating of the samples. For instance, in microwave heating, multimode-cavities are often employed which suffer from the drawback that standing waves within the cavity result in a pattern of hot and cold spots. Consequently, uniform stirring is important to avoid local overheating in hotspot areas and reduced reaction rates in cold spot areas, respectively. In cases where solid particulate substances are employed as reactants or catalysts, effective stirring can prevent sedimentation and ensure homogenous and uniform reaction conditions throughout the sample.
It is known that conditions in the sample vessel can drastically vary in the course of microwave-assisted chemical processes. For instance, an increase in sample viscosity or sedimentation during the process may result in a complete interruption of the stirring process. Especially, if magnetically driven stirring elements are employed, the stirring element immersed in the sample may stop rotating, while the magnetic actuator continues rotating. Controlling the rotation of the actuator only, may therefore lead to the false impression that stirring of the sample is still in progress. Thus, in conventional chemistry, visual inspection of the sample vessel is employed to ensure rotation of the stirring element.
In U.S. Pat. No. 6,076,957 a magnetic stirrer adapted for use with microwave ovens is described, where the sample to be heated is arranged on a turntable provided within a multimode cavity of a microwave oven. The stirring device includes a gear train assembly that increases in the normal rate of revolution of the microwave turntable by several fold and drives a magnetic actuator, which causes rotation of a magnetic stirring element immersed within the sample. In such devices, rotation of the stirring element can usually not be controlled by visual inspection. In addition, even in processes where starting viscosities and end viscosities should not pose particular problems, localized scaling or agglomerations may still stop the stirring element. Moreover, when a constantly rotating magnetic actuator is used, the stopped stirring element will usually not start-up rotating again, unless the actuator is also stopped or at least rotated with reduced speed and slowly brought up to the default rotational speed again.
Other techniques such as overhead stirrers using a drive shaft to rotate a stirring element connected to the drive shaft for aggressive chemical substances or pressure tight vessels and containers can scarcely be employed in microwave chemistry due to leakage problems, arching, surface currents etc.